JP2010068035A - Electrostatic vibrator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a structure of an electrostatic vibrator whose frequency is adjustable without employing a PLL system, and to provide a frequency adjusting method therefor. <P>SOLUTION: The electrostatic vibrator, in which both ends of vibration portions 102 and 103 of an MEMS type electrostatic drive bending vibrator having a vibration boundary condition of a both-end fixed type, and other structures are formed in one body, has a silicon oxide film formed on at least the structure, and compressive stress due to thermoelastic stress of the structure is applied to the vibration portions 102 and 103. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an electrostatic vibrator used for a reference frequency oscillator of various electronic devices.

  While there is an increasing demand for miniaturization and high precision of wireless portable devices such as mobile phones, and electronic devices such as personal computers and watches, such electronic devices have small and stable high frequency. A signal source is essential. A typical electronic component for satisfying this requirement is an AT cut crystal resonator (hereinafter simply referred to as AT cut).

  It is known that the AT cut has a very high resonance sharpness, that is, a Q value, which is an index of quality as an oscillation element, because of good crystal stability, and exceeds 10,000. This is the reason why AT cut is widely used as a stable high-frequency signal source for wireless portable devices and personal computers. However, it has become clear that this AT cut cannot be satisfactorily satisfied with respect to the recent demand for strong downsizing.

  In other words, as is well known, high-frequency electronic components other than AT-cuts have been realized with integrated formation and bonding with ICs, along with dramatic improvements in silicon MEMS (Micro-Electro-Mechanical-System) technology. One chip. However, since it is very difficult to physically bond a crystal single crystal and a silicon crystal, it is impossible to form a single chip including AT cut because it is impossible to form a single crystal or a silicon crystal. The above is the reason why the AT cut cannot sufficiently satisfy the recent demand for miniaturization.

  In order to solve this problem, a vibrator that has been attracting attention in recent years is an electrostatic vibrator using a silicon single crystal and MEMS technology. This electrostatic vibrator is a vibrator that converts a mechanical vibration having a high Q characteristic of a vibrator formed of silicon into an electric signal through an electrostatic force, and has a high Q equivalent to a quartz vibrator. It is a vibrator with very excellent characteristics that it can realize impedance characteristics with characteristics, and can realize integral formation and junction formation with ICs that cannot be realized with crystal resonators typified by AT cut ( For example, refer nonpatent literature 1).

This electrostatic vibrator is manufactured using the SOI wafer shown in FIG. FIG. 7 is a schematic diagram of the SOI wafer. The SOI wafer is a wafer having a three-layer structure of a silicon substrate 701, a silicon oxide layer formed on the silicon substrate 701, that is, a box layer 702, and a silicon layer 703 formed on the box layer 702. The electrostatic vibrator according to the present invention is formed of the silicon layer 703 shown in FIG. 7 by using MEMS technology such as DRIE.
T. Mattila et al., “14MHz Micromechanical Oscillator”, TRANSDUCERS'01 EUROSENSORS XV, The 11th International Conference on Solid-State Sensors and Actuators, Munich, Germany, 2001

  However, the conventional electrostatic vibrator has problems as described below. FIG. 8 is a perspective view showing a typical structure of a conventional electrostatic vibrator, and is a perspective view of a bending vibration type electrostatic vibrator. The fixed portion 803 and the vibrating portion 804 are integrally formed with a box layer 802 on the silicon substrate 801 interposed. The two pairs of excitation electrodes 805 are arranged on the vibration layer 804 so as to induce a bending vibration displacement 806, and are formed on the box layer 802. Incidentally, the vibration part 804 has a length dimension of L and a width dimension of a. At this time, the natural frequency F of the electrostatic vibrator is determined by the following equation (1).

ここで、E、ρはそれぞれシリコンのヤング率と密度であり、ａ、Lは、図８記載の振動子の幅寸法と長さ寸法である。

Here, E and ρ are the Young's modulus and density of silicon, respectively, and a and L are the width and length dimensions of the vibrator shown in FIG.

  When used as a stable high-frequency signal source for wireless portable devices, personal computers, etc., the natural frequency of the device must match the target frequency within several tens of ppm. In the electrostatic vibrator using silicon according to the present invention and the AT-cut quartz crystal vibrator described above, the deviation of the natural frequency from the target frequency is within tens of ppm in terms of the mechanical processing accuracy of the vibrator. Is possible. In order to solve this problem, in AT-cut quartz resonators, the excitation electrode directly formed on the vibrating part is shaved by a method such as ion milling to change the mass of the excitation electrode and finely adjust the natural frequency. ing. However, in the electrostatic vibrator according to the present invention, in the same high frequency band as the AT-cut crystal vibrator, the area of the vibration part 804 becomes very small as shown by the equation (1). It is very difficult to fine-tune the mass of 804.

  In order to solve this frequency adjustment problem, the conventional silicon electrostatic vibrator does not adjust its mechanical natural frequency, but uses a PLL (Phase Locked Loop) for the frequency output by the oscillation circuit. Thus, there has been proposed an electric circuit frequency adjustment method of controlling to a specified frequency. FIG. 9 is a block diagram showing a frequency adjusting method for this conventional electrostatic vibrator. In the frequency adjustment method shown in FIG. 9, the oscillation frequency generated in the electrostatic vibrator 901 and the oscillation circuit 902 is input to the PLL unit 903, and the frequency is adjusted based on the frequency information input in advance from the outside to obtain a signal. Is output. However, since the frequency adjustment method of the conventional electrostatic vibrator employs the PLL method, the circuit scale becomes very large, the cost of the LSI increases, and the phase noise characteristic deteriorates greatly. There were problems such as current consumption. Therefore, as described above, this electrostatic vibrator still has a very excellent feature, but is still difficult to put into practical use. That is, an object of the present invention is to provide a structure of an electrostatic vibrator capable of adjusting a frequency without adopting a PLL method and a frequency adjusting method thereof.

  The present invention has been made to solve the above-described problems, and both ends of a vibrating portion of a MEMS electrostatic drive bending vibrator having a vibration boundary condition of both ends fixed type are integrally formed with a stress applying structure. In addition, a structure in which a silicon oxide film is formed at least in the stress applying structure portion is employed.

  Thermoelasticity of the stress application structure that occurs before and after the formation of the silicon oxide film in a structure in which both ends of the vibration part and stress application structure of the MEMS electrostatic drive bending vibrator with fixed both-end vibration boundary conditions are integrally formed When the residual stress accompanying deformation is changed by trimming the silicon oxide film, the stress applied to the vibration part changes, and as a result, the natural frequency of the vibration part changes. If this vibrator structure is adopted, the area of the stress applying structure portion on which the silicon oxide film is formed can be made larger than the area of the vibrating portion, so the ratio with the spot area of trimming is relatively small. can do. Therefore, the natural frequency can be finely adjusted, and the natural frequency of the electrostatic vibrator itself can be adjusted. As a result, the circuit scale is significantly reduced as compared with the conventional PLL type frequency adjustment method, and the current consumption can also be reduced. Furthermore, since PLL control is not necessary, phase noise can be reduced.

  Hereinafter, an embodiment of the present invention and its operation will be described with reference to the drawings. FIG. 1 is a perspective view showing the structure of the electrostatic vibrator according to the first embodiment of the present invention. On the silicon substrate 101, a vibrator having a length L and a width a, which is composed of two vibrating parts having the same boundary dimensions and having a vibration boundary condition fixed at both ends, is formed. The two vibrating parts are a vibrating part 102 and a vibrating part 103, respectively.

  The vibrator is integrally formed with the fixed portion 104 and the fixed portion 105. A frequency adjustment beam 106 is formed as a second beam at an intermediate position between the vibration unit 102 and the vibration unit 103, and the frequency adjustment beam 106, the vibration unit 102, and the vibration unit 103 are connected to the fixed unit 104 and the fixed unit 105. It is formed integrally with interposition. The fixing portion 104 is fixed on the silicon substrate 101 with a box layer 107 interposed therebetween. On the other hand, a gap 108 corresponding to the thickness of the box layer 107 exists between the fixed portion 105 and the silicon substrate 101.

  The vibration displacements of the vibration unit 102 and the vibration unit 103 are a vibration displacement 109 and a vibration displacement 110, respectively, and their vibration amplitudes are equal to each other and their phases are different by 180 °. Excitation electrodes for exciting such vibration displacements are the excitation electrode 111, the excitation electrode 112, the excitation electrode 113, and the excitation electrode 114 shown in the figure. These excitation electrodes are also provided on the silicon substrate 101 with the box layer 107 interposed therebetween. And is integrally formed. The excitation electrode 111 and the excitation electrode 113 have the same polarity, and the excitation electrode 112 and the excitation electrode 114 have an inverted polarity with respect to the excitation electrode 111 and the excitation electrode 113. In the manufacturing process of the electrostatic vibrator according to the present invention, at least the frequency adjustment beam 106 is formed with a silicon oxide film.

  In the electrostatic vibrator shown in the figure, the natural frequency of the vibration unit 102 and the vibration unit 103 is adjusted by selectively trimming the silicon oxide film formed on the frequency adjustment beam 106 using ion milling or the like. I can do it. The gap 115 provided in the silicon substrate 101 is a gap used when removing the oxide film formed on the frequency adjustment beam 106, and the oxide film on the back surface of the frequency adjustment beam 106 is removed by ion milling or the like. This is a necessary gap.

  FIG. 2 is a diagram for explaining a mechanism in which thermoelastic deformation is applied as a compressive stress to the vibrating portion in the electrostatic vibrator shown in FIG. 1 according to the present invention. This figure shows the vibration part 102, the vibration part 103 according to the present invention shown in FIG. 1, and is formed at an intermediate position between the vibration parts, the fixed part 104, and the fixed part 105, and the silicon oxide film. It is a front view of the electrostatic vibrator which consists of the frequency adjustment beam 106 with which was formed. In this figure, by trimming the silicon oxide film formed on the frequency adjustment beam 106 with the ion beam 203, a thermoelastic deformation stress 201 is generated in the frequency adjustment beam 106, and at the same time, in the vibration unit 102 and the vibration unit 103, The vibration part thermoelastic stress 202 is generated. At this time, the natural frequency F of the electrostatic vibrator shown in FIG. 1 is approximately determined by the following equation (2).

ここで、Ｅ、ρはそれぞれシリコンのヤング率と密度であり、Ｗ、ａは、図１記載の振動子の幅寸法と長さ寸法である。さらにσ₀は図２記載の熱弾性応力２０２の大きさである。この応力は圧縮応力であるので負の値である。またｋは比例定数であって、振動部におけるシリコン酸化膜の有無によってその大きさは相違するが、本発明の作用には大きく影響しないので詳細説明は省略する。

Here, E and ρ are the Young's modulus and density of silicon, respectively, and W and a are the width and length dimensions of the vibrator shown in FIG. Further, σ ₀ is the magnitude of the thermoelastic stress 202 shown in FIG. Since this stress is a compressive stress, it is a negative value. Further, k is a proportional constant, and the size thereof varies depending on the presence or absence of the silicon oxide film in the vibration part, but since it does not greatly affect the operation of the present invention, detailed description thereof is omitted.

FIG. 3 is a characteristic diagram showing the relationship between the magnitude of the thermoelastic stress 202 shown in FIG. 2 and the trimming amount of the silicon oxide film. The vertical axis represents the magnitude σ ₀ of the thermoelastic stress 202 and the horizontal axis represents the silicon oxide. This is a film trimming amount M. This figure is a characteristic diagram in the case where a silicon oxide film is formed on the vibrating portion 102, the vibrating portion 103, and the frequency adjusting beam 106 shown in FIG. In this case, before trimming, that is, when the trimming amount is 0, no thermoelastic stress is generated, and as the trimming amount increases, the thermoelastic stress increases to the negative side. The characteristic curve 301 shows this behavior. In the thermal firing, a silicon oxide film is also formed on the surfaces of the silicon substrate 101, the excitation electrode 111, the excitation electrode 112, the excitation electrode 113, and the excitation electrode 114 other than the vibration part and the frequency adjustment beam according to the present invention. The frequency adjustment, which is a problem of the present invention, does not have a great influence, and therefore the illustration of this figure is omitted.

FIG. 4 is a second characteristic diagram showing the relationship between the magnitude of the thermoelastic stress 202 shown in FIG. 2 and the trimming amount of the silicon oxide film. The vertical axis represents the magnitude σ ₀ of the thermoelastic stress 202 and the horizontal axis. Is the trimming amount M of the silicon oxide film. This figure is a characteristic diagram when a silicon oxide film is formed only on the frequency adjustment beam 106 shown in FIG. In this case, the silicon oxide film is formed by a thin film forming method such as CVD. In this thin film formation, the magnitude of the aforementioned thermoelastic stress changes depending on the temperature conditions during film formation, but this is merely a process condition setting item.

In this figure, before trimming, that is, when the trimming amount is 0, the thermoelastic stress is a negative value σ ₁ , and as the trimming amount increases, the thermoelastic stress increases to the positive side and the silicon oxide film is completely removed. When removed, its value becomes zero. It is the characteristic curve 401 that shows this behavior. From the above characteristic curve 301 and characteristic curve 401 and the equation (2), the electrostatic vibrator according to the present invention has a frequency when the silicon oxide film formed on the frequency adjustment beam 106 shown in FIGS. 1 and 2 is trimmed. It turns out that changes. That is, when the silicon oxide film is formed on the vibration unit 102, the vibration unit 103, and the frequency adjustment beam 106 illustrated in FIG. 1, the natural frequency is reduced by trimming the silicon oxide film, and only the frequency adjustment beam 106 illustrated in FIG. In the case where a silicon oxide film is formed, the natural frequency is increased by trimming the silicon oxide film. Since the area of the frequency adjustment beam on which the silicon oxide film according to the present invention is formed can be made larger than the area of the vibration part, the ratio of the trimming beam to the spot area can be made relatively small. Therefore, the natural frequency can be finely adjusted, and the natural frequency of the electrostatic vibrator itself can be adjusted.

  FIG. 5 is a perspective view showing the structure of the electrostatic vibrator according to the second embodiment of the present invention. The silicon substrate 501 is provided with a vibrator having a length L and a width a, which is composed of two vibration parts having the same boundary dimensions and the vibration boundary condition being fixed at both ends. These two vibrating parts are respectively. The vibration unit 502 and the vibration unit 503. The vibrator is integrally formed with a fixed portion 504 and a fixed portion 505. A pair of frequency adjustment beams 506 is formed as a second beam in a position parallel to the vibration unit 502 and the vibration unit 503, and the frequency adjustment beam 506, the vibration unit 502, and the vibration unit 503 are fixed. It is integrally formed with a part 504 and a fixed part 505 interposed. The fixing portion 504 is fixed on the silicon substrate 501 with the box layer 507 interposed therebetween. On the other hand, a gap 508 corresponding to the thickness of the box layer 507 exists between the fixed portion 505 and the silicon substrate 501. The vibration displacements of the vibration part 502 and the vibration part 503 are vibration displacement 509 and vibration displacement 510, respectively, and their vibration amplitudes are equal to each other and their phases are different by 180 °.

  Excitation electrodes for exciting such vibration displacement are the excitation electrode 511 and the excitation electrode 512 shown in the figure, and both the excitation electrodes are integrally formed with the silicon substrate 501 with the box layer 507 interposed therebetween. The pair of excitation electrodes 511 outside the vibration part 502 and the vibration part 503 and the excitation electrodes 512 inside the both vibration parts are electrically connected so as to have different polarities. In the manufacturing process of the electrostatic vibrator according to the present invention, a silicon oxide film is formed on at least the pair of frequency adjustment beams 506.

  FIG. 6 is a diagram for explaining a mechanism in which thermoelastic deformation is applied as a compressive stress to the vibrating portion in the electrostatic vibrator according to the present invention shown in FIG. This figure is a front view of an electrostatic vibrator comprising a vibrating portion 502, a vibrating portion 503, and a pair of frequency adjustment beams 506 integrally formed in parallel positions with the vibrating portion, a fixing portion 504, and a fixing portion 505 interposed therebetween. FIG. In this figure, by trimming the silicon oxide film formed on the frequency adjustment beam 506 with the ion beam 603, a thermoelastic deformation stress 601 is generated in the frequency adjustment beam 506, and at the same time, the vibration unit 502 and the vibration unit 503 are also affected. A vibration portion thermoelastic stress 602 is generated. At this time, the natural frequency F of the electrostatic vibrator shown in FIG. 5 is determined by the above equation (2) as in the electrostatic vibrator shown in FIG.

  In the case where the silicon oxide film is formed on the vibrating unit 502, the vibrating unit 503, and the frequency adjusting beam 506 shown in FIG. 5 by thermal baking, the vibrating unit 502 is trimmed before the frequency adjusting beam 506 is trimmed, that is, when the trimming amount is zero. In addition, no thermoelastic stress is generated in the vibrating portion 503, and the thermoelastic stress increases to the negative side as the trimming amount increases. This behavior is the same as the characteristic curve 301 shown in FIG. 3 for explaining the mechanism of the first embodiment according to the present invention. In the thermal firing, a silicon oxide film is also formed on the surface of the silicon substrate 501, the excitation electrode 511, and the excitation electrode 512 other than the vibration part and the frequency adjustment beam according to the present invention. The improvement of the temperature characteristic does not have a great influence, and therefore the illustration is omitted for the explanation of this figure. In addition, when the silicon oxide film is formed only on the frequency adjustment beam 506 shown in FIG. 5 by a thin film formation method such as CVD, the thermoelastic stress is a negative value when the trimming amount of the frequency adjustment beam 506 is zero. As the trimming amount increases, the thermoelastic stress increases to the positive side, and the value becomes 0 when the silicon oxide film is completely removed. This behavior is the same as the characteristic curve 401 shown in FIG. 4 for explaining the mechanism of the first embodiment according to the present invention. In the thin film formation according to this figure, the magnitude of the thermoelastic stress described above varies depending on the temperature conditions during film formation. However, as in the explanation of FIG.

  As described above, the embodiments of the present invention have been described in detail with reference to the drawings. However, the specific configuration is not limited to these embodiments, and includes design changes and the like without departing from the gist of the present invention. . That is, in an electrostatic vibrator in which both ends of a vibrating portion of a MEMS electrostatic drive bending vibrator having a fixed both-end vibration boundary condition and another structure are integrally formed, at least the structure has silicon oxide. By adopting a structure in which a compressive stress due to the thermoelastic stress of the structure is applied to the vibrating part while forming a film, an electrostatic vibrator capable of adjusting the frequency without adopting a PLL method Structure can be provided.

The perspective view which shows the structure of the electrostatic vibrator by 1st embodiment which concerns on this invention. The figure for demonstrating the compressive-stress application mechanism in 1st embodiment which concerns on this invention. 2 is a characteristic diagram showing the relationship between the magnitude of the thermoelastic stress 202 shown in FIG. 2 and the trimming amount of the silicon oxide film. Second characteristic diagram showing the relationship between the magnitude of the thermoelastic stress 202 shown in FIG. 2 and the trimming amount of the silicon oxide film The perspective view which shows the structure of the electrostatic vibrator by 2nd embodiment of this invention. The figure for demonstrating the compression stress application mechanism in 2nd embodiment of this invention Conceptual diagram of SOI wafer for manufacturing of electrostatic vibrator Perspective view of a single-end fixed bending vibration type electrostatic vibrator showing the structure of a conventional electrostatic vibrator Block diagram showing frequency adjustment method of conventional electrostatic vibrator

Explanation of symbols

101, 501, 701, 801 Silicon substrate 102, 103, 502, 503, 804 Vibrating part 104, 105, 504, 505, 803 Fixed part 106, 506 Frequency adjustment beam 107, 507, 702, 802 Box layer 108, 508 Air gap 109, 110, 509, 510 Vibration displacement 111, 112, 113, 114, 511, 512, 805 Excitation electrode 115 Cavity 201, 202, 601, 602 Thermoelastic deformation stress 203, 603 Ion beam 301, 401 Characteristic curve 703 Silicon Layer 806 Bending vibration displacement

Claims (5)

  In an electrostatic vibrator in which both ends of a vibrating portion of a MEMS electrostatic drive bending vibrator having a fixed both-end vibration boundary condition and another structure are integrally formed, at least the structure has a silicon oxide film. A MEMS electrostatic vibrator having a structure in which a compressive stress caused by a thermoelastic stress of the structure is applied to the vibrating portion.   A second beam having an oxide film formed at a position parallel to the vibrating portion; both ends of the beam and both ends of the vibrating portion are integrally formed; and only one end of the vibrating portion is silicon The electrostatic vibrator according to claim 1 connected on a substrate.   The electrostatic vibrator according to claim 1, wherein the silicon oxide film is formed of a heat-fired film.   The electrostatic vibrator according to claim 1, wherein the silicon oxide film is formed of a CVD film.   5. The electrostatic vibrator according to claim 1, wherein the natural frequency of the vibrator is adjusted by removing the oxide film formed on the second beam by ion milling or the like. 6.

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